The processing of powder materials through the in-flight melting of the individual particles under plasma conditions followed by the solidification of the formed droplets has been known for some time and is attracting increasing attention as a means of densification and spheroidisation materials in powder form. The process, generally known as powder spheroidisation, results in a significant improvement of the flow properties of the powders, and the increase of their resistance to attrition during their handling and transport.
The powder spheroidisation process has also been recognized as an effective means for the proper control of the chemical composition of the powder materials as well as for the synthesis of new materials and composite mixtures.
Through the use of inductively coupled, radio frequency (r.f.) electrodless discharges, as a heat source for the process, it has also been observed that the process can be used for the significant purification of the powder being treated through the partial loss of some of the impurities either as a result of a simple volatilization step from the molten droplets, or the reactive volatilization of the impurities. In the former case, the impurities of lower boiling point compared to that of the particle matrix are preferentially vaporized; the gaseous impurities can escape from the particle matrix. In the latter case, the impurity is chemically transformed at the surface of the molten droplet through its contact with the processing environment, followed by the volatilization of the formed compound. The chemical reaction involved can be, though not limited to, for example, the oxidation of the impurities through their contact with oxygen in the plasma flow. The process results in a net reduction of the level of impurities in the powder and subsequently its purification.
The problem that arises in such circumstances, however, is that the formed vapour cloud of the impurities, whether they are in their elemental form, or as a compound, remains mixed with the plasma gas transporting the purified powder. As the overall plasma stream with its powder content is cooled down, the impurities also condenses in the form of a very fine soot that deposits on all available surfaces in the reactor including the surface of the processed/purified powders which are then contaminated again with the same impurities that were eliminated in the first place. In the case of metal powder, this soot is composed of very fine metallic particle. These fine particles are, in turn, very sensitive to oxidation when they come in contact with the ambient air, with which they react, resulting in the significant increase of the oxygen content of the powder.
In a different context, the induction plasma processing of powders has also been successfully used for the synthesis of metallic and ceramic nanopowders through the in-flight heating, melting and vaporization of the feed precursor followed by the rapid quench of the formed vapours in order form a fine aerosol of nanopowder thorough the homogenous condensation of the vapour cloud. In such a case, however, the formed aerosol of nanopowder is mixed with residual fraction of the feed material, which is only partially vaporized, resulting in a mixed powder with a broad particle size distribution. Depending on the operating conditions, the collected powder can often have a bimodal particle size distribution, which represents a major limitation to the acceptance of such a powder for most nanopowder applications.